When you say CGEM do you mean any EQ style mount or specifically the Celestron CGEM EQ mount? The OTA from an 8SE will fit on a CGEM mount, but you need to add a "D" style dovetail. The 8SE OTA will mount on a CG-5 EQ mount without any modification.

which allows you to attach both the CGE/D/Losmandy (wide) and the CG-5/V/Vixen (skinny) dovetails. That way you can attach most any OTA (optical tube assembly) to your mount. Otherwise you would need to get the larger dovetail and attach it to the OTA, like this:

CGEM is Celestron's Computerized German Equatorial Mount. They have two of them, the standard CGEM and the CGEM-DX. Either one can easily handle the 8SE OTA. The tripod that comes with the CGEM-DX is way heavier than the one that comes with the standard CGEM.

Like Stan says, it sounds like you might be using "CGEM" generically to mean an equatorial mount? A CGEM is a specific mount made by celestron. "GEM" is the generic term to mean a German equatorial Mount.

Your 8SE OTA (optical tube assembly) can be mounted on any GEM that has a vixen style dovetail seat ... and most of them do.

Before recommending a specific mount, it would be helpful to know if you want to use it for visual or astrophotography.

If you can swing the CGEM and the weight (40#+/-) isn't an issue, go for one. Used about 2X the price of a CG-5, but worth it IMO.

ADM also makes an adapter that fits the "D" (3" wide) and then accepts the Vixen format 1.7" wide dovetail that is fitted to your SE C8. Adapter runs about $100. A Celestron D spec dovetail for the SE C8 can be had for $30 on eBay, but then you lose the ease of use on the SE mount.

Yes a nexstar 8 OTA can be placed on a CGEM. I have put my 8 inch OTA on CG5 and on a CGEM once and out of despiration on a vixen porta 2 mount. I would really not recommend the latter. It took a long time to settle down or did it?